FIG. 1 shows an example of configuration of a typical wavelength multiplex transmission system (e.g. see non-patent literatures 1 to 8). This is an example where three wavelengths are wavelength-multiplexed. Three electrical signals 7a, 7b and 7c inputted from input terminals 191, 192 and 193, are converted to optical signals 7A, 7B and 7C with different wavelengths by optical transmitters 111, 112 and 113, respectively. These optical signals are wavelength-multiplexed by wavelength multiplex filter 131 and converted to one wavelength-multiplexed signal, which is transmitted via optical transmission line 151. The transmitted wavelength-multiplexed signal is separated by wavelength separation filter 132 to optical signals with their respective wavelengths, which are outputted from output terminals 194, 195 and 196 by optical receivers 121, 122 and 123, respectively.
In such wavelength multiplex transmission system 171, crosstalk between wavelengths can occur at optical transmission line 151 or wavelength separation filter 132. When crosstalk occurs, crosstalk components are superposed on the signal, and this can lead to deterioration of the optical signal (e.g. see non-patent literatures 1 to 7).
As an example, FIG. 2 shows flows of signals when three wavelengths are wavelength-multiplexed. Signals 7a, 7b and 7c are electrical signals inputted into wavelength multiplex transmission system 171. These electrical signals are inputted from input terminals 191 into optical transmitter 111, inputted from input terminals 192 into optical transmitter 112, and inputted from input terminals 193 into optical transmitter 113, respectively. Optical signals 7A, 7B and 7C are outputted from the respective optical transmitters and transmitted. In transmission of the optical signals, if there is no crosstalk among wavelengths of optical signals 7A, 7B and 7C, then signals 7a, 7b and 7c are outputted from optical receivers 121, 122 and 123, respectively.
FIG. 3 shows flows of signals in a three-wavelength multiplexed transmission system when there is crosstalk in transmission of optical signals. Signals 7a, 7b and 7c are electrical signals inputted into wavelength multiplex transmission system 171. These electrical signals are inputted from input terminal 191 into optical transmitter 111, inputted from input terminal 192 into optical transmitter 112, and inputted from input terminal 193 into optical transmitter 113, respectively. Optical signals 7A, 7B and 7C are outputted from the respective optical transmitters and transmitted. When there is crosstalk among wavelengths of optical signals 7A, 7B and 7C, crosstalk components, in addition to electrical signals 7a, 7b and 7c, are outputted from optical receivers 121, 122 and 123, respectively. That is, from output terminal 194 of optical receiver 121, electrical-level crosstalk components 9ba and 9ca from optical signals 7B and 7C are outputted in addition to signal 7a. From output terminal 195 of optical receiver 122, electrical-level crosstalk components 9ab and 9cb from optical signals 7A and 7C are outputted in addition to signal 7b. From output terminals 196 of optical receiver 123, electrical-level crosstalk components 9ac and 9bc from optical signals 7A and 7B are outputted in addition to signal 7c. Here, reference symbols 9BA, 9CA, 9AB, 9CB, 9AC and 9BC in the figure denote optical crosstalk components.
Crosstalk among wavelengths can be caused by stimulated Raman scattering (SRS), cross phase modulation (XPM) and the like, which are due to nonlinearity of optical fiber constituting the optical transmission line (e.g. see non-patent literatures 2, 3, 5 and 7). Crosstalk among wavelengths can also be caused by poor wavelength separation characteristic of the wavelength separation filter, in addition to the nonlinearity of optical fiber (e.g. see non-patent literature 4).
As a method for reducing such crosstalk, crosstalk can be reduced by making polarization directions of adjacent optical signals orthogonal when multiplexing and transmitting over an optical transmission line (e.g. see patent literature 1). However, this method is effective only for adjacent wavelengths, and no effect of reducing crosstalk has been obtained for non-adjacent wavelengths.
[Patent Literature 1] Japanese Patent Application Laid-Open No. 08-18536
[Non-Patent Literature 1] K. Kikushima et al., “Signal crosstalk due to fiber nonlinearity in wavelength multiplex SCM-AM TV transmission systems,” Optical Fiber Communication Conference (OFC '95), Post-deadline paper, PD24, February to March 1995
[Non-Patent Literature 2] A. Li et al., “Experimental confirmation of crosstalk due to stimulated Raman scattering in WDM AM-VSB transmission systems,” Electronics Letters, vol. 31, No. 18, pp. 1538-1539, August 1995
[Non-Patent Literature 3] Takachio et al., “Review of 10 Gb/s 8CH WDM Transmission System in Arrangement of Wavelengths at Irregular Intervals,” Technical Research Report of the Institute of Electronics, Information and Communication Engineers, CS96-43, pp. 19-24, June 1996
[Non-Patent Literature 4] K-P Ho et al., “Demultiplexer crosstalk rejection requirements for hybrid WDM system with analog and digital channels,” IEEE Photonics Technology Letters, Vol. 10, No. 5, pp. 737-739, May 1998
[Non-Patent Literature 5] M. R. Phillips et al., “Crosstalk due to optical fiber nonlinearities in WDM CATV lightwave systems,” IEEE Journal of Lightwave Technology, Vol. 17, No. 10, pp. 1782-1792, October 1999
[Non-Patent Literature 6] Shibata et al., “Optical Picture Delivery System With FM Batch Conversion Method,” The Journal of the Institute of Electronics, Information and Communication Engineers, B, Vol. J. 83-B, No. 7, pp. 948-959, July 2000
[Non-Patent Literature 7] F. Coppimger et al., “Nonlinear Raman crosstalk in a video overlay passive optical networks,” Optical Fiber Communication Conference (OFC 2003) TuR5, March 2003
[Non-Patent Literature 8] ITU-T Recommendation G. 983.3 “A broadband optical access system with increased service capability by wavelength allocation”